The present invention relates to a method of chromatographic separation, in which a feedstock fluid and a desorbent fluid are supplied into an adsorbent-packed bed, and wherein, as the feedstock fluid moves through the bed, the components in said fluid are separated by interaction with the adsorbent. At least two fluids, one being rich in a component that interacts strongly with the adsorbent and the other being rich in a component that interacts weakly with the adsorbent, are extracted from the bed.
Chromatographic separation is one separation technique practiced extensively in industrial applications. While several techniques of chromatographic separation are known, a simulated moving-bed system has been used most extensively in large-scale operation. In such a system, a fluid stream circulates in the same direction through a bed packed with an adsorbent. The bed is provided with more than one supply inlet and more than one extraction outlet. More specifically, a feedstock fluid supply inlet, a nonadsorbate fluid extraction outlet, a desorbent fluid supply inlet, and an adsorbate fluid extraction outlet are arranged, in that order, in the direction of fluid flow. During the separating operation, one supply inlet/extraction outlet pair is always in an active state. As a predetermined working time lapses, this active pair is made inactive, and the next corresponding pair, which is located immediately downstream, is made active. Therefore, when as many switching operations as there are supply inlet/extraction outlet pairs in the bed have been effected, the active supply inlet/extraction outlet pair is the same as just before the first switching operation was performed.
For the purpose of the following discussion, the zone from a certain supply inlet or extraction outlet to a corresponding supply inlet or extraction outlet that is located immediately downstream will be referred to as "a unit packed bed." A packed beds can be regarded as a series connection of as many unit packed beds as the number of supply inlet/extraction outlet pairs employed. The "feedstock fluid supply inlet", "desorbent fluid supply inlet", "nonadsorbate fluid extraction outlet", and "adsorbate fluid extraction outlet" are named based on the function of an apparatus for supplying a fluid into or extracting it from the bed, and in practice, a single apparatus may have the ability to perform more than one of the four functions of interest. In fact, it is common practice for a single apparatus to serve both as a feedstock fluid inlet and as a desorbent fluid supply inlet, or serve both as a nonadsorbate fluid extraction outlet and as an adsorbate fluid extraction outlet.
With respect to an active supply inlet/extraction outlet pair, the zone between a feedstock fluid supply inlet and a nonadsorbate fluid extraction outlet is referred to as an adsorption zone; the zone between the nonadsorbate fluid extraction outlet and a desorbent fluid supply inlet is referred to as a refining zone; the zone between the desorbent fluid supply inlet and an adsorbate fluid extraction outlet is referred to as a desorption zone; and the zone between the adsorbate fluid extraction outlet and the feedstock fluid supply inlet is referred to as a concentration zone. Therefore, the packed bed consists of four zones, each zone normally containing a plurality of unit packed beds.
Each of the components to be separated in the packed bed has a particular concentration distribution in the direction of feed flow. This concentration distribution will move downstream through successive zones of the bed while retaining its shape. One supply inlet/extraction outlet pair is switched to another pair in synchronism with the movement of concentration distributions so that a fluid can be supplied to a desired position on a particular concentration distribution while another fluid is extracted from another desired position.
In the basic operation of a simulated moving bed, feedstock fluid and a desorbent fluid may be supplied into the bed through certain supply inlets at any point of time, and a nonadsorbate fluid and an adsorbate fluid are extracted through certain extraction outlets. As a result, the operation can be regarded as being continuous on the whole with respect to the supply and extraction of fluids. The fluid to be extracted is only part of the fluid that reaches a cross section of the bed at which the outlet through which it is extracted is positioned, and the greater part of the fluid will move downstream without being extracted.
Normally, an amount of fluid equal to 4-10 times the volume of the fluid that is supplied into each zone from the outside of the bed, or which is extracted from each zone to the outside of the bed, will flow into each zone from the zone located upstream thereof. Therefore, a particular concentration distribution that is formed in the bed is capable of moving downstream without being greatly distorted in spite of fluid extraction from the bed.
As described above, a supply inlet/extraction outlet pair is switched to a pair immediately downstream in synchronism with the downstream movement of a particular concentration distribution formed in the bed. Although movement of a concentration distribution is continuous, switching between adjacent supply inlet/extraction outlet pairs is intermittent, resulting in a time-dependent change in the composition of a fluid being extracted through one outlet within a unit working time. In order to attain better separation performance, it is preferable that the fluid being extracted should experience the smallest possible change in composition as a function of time. To this end, the duration for which a supply inlet/extraction outlet pair is in active operation must be shortened, and frequently must be switched to successive supply inlet/extraction outlet pairs located downstream of the bed. However, such requires a bed composed of many unit packed beds, and so the overall equipment becomes complex and costly. In consideration of cost and desired separation efficiency, the equipment typically used consists of 6-24 unit packed beds.